1. Field of the Invention
The invention relates to turbomachines, such as aircraft turbojet engines, in which the bearings are permanently lubricated by liquid lubricant feed circuits during operation. Such a circuit normally includes a liquid lubricant reservoir, and a pump for delivering the lubricant to a nozzle situated in the immediate vicinity of each of the bearings. The bearings are mounted in lubrication chambers which are closed by seals and from the, bottom of which a duct leads for the recovery of an emulsion consisting of the liquid lubricant mixed with air. Incorporated in this duct is a recovery pump which delivers the above-mentioned emulsion to a liquid lubricant/air separator after first having been cooled in a suitable cooling device. From the separator the cooled liquid is returned to the reservoir, while the air is evacuated directly to the outside through an exhaust port.
The lubrication chambers surrounding the bearings in a turbomachine are located in areas which are at an excess pressure relative to the internal pressure of these chambers under normal operational conditions of the turbomachine, the excess pressure usually being provided by the compressor of the turbomachine. This arrangement normally prevents any escape of liquid lubricant to the outside of the lubrication chambers.
However, when the operating speed of the turbomachine is below its idling rate, the pressure normally prevailing outside the lubrication chambers may come close to the pressure prevailing inside the chambers, and may even fall below this pressure. Such conditions may particularly occur when there is a considerable fall in the rotational speed for whatever reason, when the machine cuts out and rotates only in self-rotation, when attempts at restarting the machine are made from stop, and when the machine operates without ignition.
Such a reduction in the pressure which normally prevails outside the lubrication chambers of the bearings in a turbomachine causes leakage of lubricant towards the outside of the lubrication chambers. This leakage results in a pollution of the turbomachine and of its immediate environment, which can be seen from the outside. On an aircraft, such leakage results in depletion of the necessarily reduced reserve of lubricant liquid, which can put at risk the engines and, consequently, the aircraft.
2. Summary of the Prior Art
To solve this problem, it is known to fit the lubrication system of a turbomachine with a device for depressurizing the lubrication chambers of the bearings, such as described in FR-A-2 536 120. This device establishes a communication between each chamber and the outside, and maintains in these chambers a pressure equal to the outside pressure increased only by the pressure drop due to the flowing of the air-lubricant emulsion in the device. This device normally comprises a duct, termed a venting duct, connected to each chamber, at least one air-lubricant separator which is usually that of the turbomachine, and an exhaust duct extending from the separator.
However, the device remains ineffective at the low speeds of the turbomachine mentioned earlier, as the compressor no longer provides excess pressure around the lubrication chambers;.